A wireless sensor system is one wherein sensors communicate wirelessly with a master unit. The master unit may in turn communicate with a stationary desktop unit or the like. The wireless connection allows for enhanced mobility, however, it also causes some inevitable data loss if an unreliable wireless transmission link is utilized. This problem is especially critical for cases where the actual quantity of interest for the final application is obtained by further sensor data processing.
This is for example the case when the wireless sensing units contain, among others, inertial sensors (accelerometers and gyroscopes). In this case, the sensor readings are acceleration and angular velocity in sensor frame, whereas the final application generally employs the integral quantities of orientation, velocity, or position, expressed in a global reference frame. These integral quantities are typically calculated by solving differential equations of motion starting from acceleration and angular velocity signals, usually within a sensor fusion framework which further processes data from other sensors, e.g., magnetometers, pressure sensors, GPS, WiFi, etc.
In many applications, it is not practical to run the complex sensor fusion algorithms directly on the wireless sensor, since such algorithms pose demands in terms of memory, processing requirements and power consumption that may overtax small battery-powered wireless sensors, e.g., those embedded in wearable systems. In some applications, it is necessary to run sensor fusion algorithms at an application processor side, in order to facilitate combination of the inertial sensor readings with additional information from, e.g., map-matching, WiFi, GSM networks and the like, which can only be gathered from higher layer applications to which the wireless sensing lacks direct access.
In either case, it is necessary to stream inertial data from the wireless sensor to a master unit through a wireless link. If this is performed by simply transmitting the direct accelerometer and gyroscope readings, the inevitable occurrence of packet loss will result in inaccurate integral values. This in turn typically requires that the inertial quantities be re-initialized, introducing errors, instability, and inconsistency, and decreasing for overall performance.
In addressing the aforementioned limitation, US 2011/0109438 A1 discloses a reliable wireless communication between a master unit and a sensor unit. According to the method disclosed therein, a sensor unit senses acceleration and angular velocity samples and then further calculates velocity and orientation increments representing their corresponding integral quantities within a time interval. The communication between the master unit and the sensor unit is initiated by the master unit with a data update request message. The data update request message additionally includes an identification of an initial sample sequence number from which the integration of acceleration and angular velocity into velocity and orientation increments should be calculated at the sensor unit. Upon receipt of the data update request message, the sensor unit resets the integration interval starting time according to the sequence number provided by the master unit, and further transmits its last-calculated integrals of acceleration and angular velocity. The identification of initial sample sequence number transmitted from the master unit to the sensor unit indicates the last correctly received packet at the master unit side, and it therefore serves as means for acknowledgement for actual packet reception.
The mechanism described in US 2011/0109438 A1 therefore offers the desired robustness to occasional packet loss introduced by the wireless link. In fact, if no data packet loss occurs, the master unit will provide an acknowledgment to the sensor unit after every single received sample, and the sensor unit will reset at each of these samples the start time for the window in which integration of acceleration and angular velocity is performed. Similarly, if for example a single transmitted sample from the sensor unit is lost, the sensor unit will not receive the corresponding acknowledgment from the master unit and it will simply continue to integrate acceleration and angular velocity during the next sample as well. Such new sample will be transmitted in the next corresponding transmission slot, and it will contain the overall integral from the last acknowledgment received from the master unit (i.e. the integral will cover in this specific case a time window corresponding to the last two acceleration and angular velocity samples). In this way, even if packet loss occurred, no integral quantity is lost even during real-time operation; sensor fusion algorithms at an application processor side can therefore continue to run without any noticeable impairment in performance (there will only be a coarser time resolution in data representation).
While the foregoing system remains ideal for many situations and is better than most available solutions, there may exist situations wherein it is desired to eliminate the use of acknowledgements from the master unit to the sensor unit. The inventors have observed that eliminating acknowledgements will serve to reduce or eliminate the need for certain detailed record keeping at the sensor unit. Moreover, the reduction in parsing and processing data messages from the master unit reduces processing requirements and the time allocated within each frame to perform said processing, increasing the overall wireless transmission efficiency.
While for some cases the additional record keeping and processing loads do not pose significant limitations for practical use, in applications with tight constraints in terms of used memory, power consumption and cost, e.g., wearable devices and accessories, savings in these areas can be important to system performance and unplugged runtime.
This Background section is provided as a convenience to the reader before reading the detailed disclosure, and it is hoped that this section was helpful. However, the contents of this section are not a substitute for prior art, and do not represent a search for prior art. All discussion in this section, including that regarding the listed publication, represent the inventors own observations and are expressly disclaimed as prior art; they do not constitute prior art nor are they an admission as to prior art. To the extent that the contents of the noted publication are of interest and a full and accurate understanding of that reference is needed, then the reader must refer to the publication itself.